Past research on daytime radiative cooling,
while successful in reducing cooling energy
consumption, typically used materials with
fixed, cooling-optimized properties, which ef-
ficiently emit thermal radiation even when the
temperature of the surface is lower than de-
sired, such as during the night or in the winter.
This unwanted thermal radiative cooling will
increase the energy consumption for heating
and may offset the cooling energy saved in hot
hours or seasons. This issue is well acknowl-
edged by the research community, and mitiga-
tion of the overcooling has become a timely
demand ( 15 ). To cut the heating penalty from
overcooling, a few techniques were recently
attempted for switching off thermal radiative
cooling at low temperatures (below 22°C).
Although effective in switching, these tech-
niques typically require either additional en-
ergy input ( 16 , 17 ) or external activation ( 18 ),
and in some cases, switching is achieved by
mechanical moving parts ( 19 , 20 ). Develop-
ing dynamic structures that automatically
cease radiative cooling at low temperatures
is therefore highly desirable. Existing efforts
in self-switching radiative cooling, however,
are either purely theoretical ( 21 – 24 ) or limited
to materials characterization with little re-
levance to practical household thermal regula-
tion ( 25 – 28 ). Very recently, a smart subambient
coating was developed ( 29 ), focusing on the
reduction of solar absorption by fluorescence
rather than modulation of thermal emittance
by temperature.
We took a different, holistic approach by de-
signing and fabricating a mechanically flex-
ible coating structure to minimize total energy
consumption through the entire year. This
temperature-adaptive radiative coating (TARC)
automatically switches its sky-window emit-
tance to 0.90 from 0.20 when the surface tem-
perature rises above ~22°C, a practical threshold
not previously available. Our TARC delivers
high radiative cooling power exclusively for
the high-temperature condition (Fig. 1A). We
also optimized the solar absorptance at ~0.25
(solar reflectance = 0.75) for all-season en-
ergy saving in major US cities (fig. S7). Our
TARC demonstrates effective surface tem-
perature modulation in an outdoor test
environment. We performed extensive sim-
ulations based on the device properties and
the climate database, which show advantages
of TARC over existing roof coating mate-
rials in energy savings for most US cities in
different climate zones (Fig. 1C). The energy
savings by TARC not only bring economic
benefits but also contribute to environmental
preservation by reducing greenhouse gas
emissions.
We developed the TARC based on the well-
known metal-insulator transition (MIT) of the
strongly correlated electron materials WxV1-xO 2
( 30 – 32 ), and the transition temperature (TMIT)
is tailored to ~22°C by setting the composi-
tionxat 1.5% ( 33 ). We embedded a lithograph-
ically patterned two-dimensional array of thin
WxV1-xO 2 blocks in a BaF 2 dielectric layer that
sits on top of an Ag film (Fig. 2A). In the in-
sulating (I) state of WxV1-xO 2 atT<TMIT, the
material is largely transparent to the infrared
(IR) radiation in the 8- to 13-mm sky spectral
window, so this sky-window IR radiation is
reflected by the Ag mirror with little absorp-
tion ( 34 ). By contrast, the WxV1-xO 2 becomes
highly absorptive in the sky window when it
switches to the metallic (M) state atT>TMIT
( 34 ). The absorption is further amplified by
the designed photonic resonance with adja-
cent WxV1-xO 2 blocks as well as with the bot-
tom Ag layer through the ¼-wavelength cavity.
The ¼-wavelength cavity structure induces
Fabry-Perot resonance and was used in prev-
ious work to enhance thermal emission ( 21 , 23 ).
According to Kirchhoff’s law of radiation ( 35 ),
the sky-window emittance equals the sky-
window absorptance and switches from lowto high when the temperature exceedsTMIT.
Consequently, strong sky-window radiative
cooling is turned on in operation exclusively
at high temperatures, leaving the system in
the solar-heating or keep-warm mode at
low temperatures. Details on the fabrica-
tion process and structural parameters are
found in the supplementary materials ( 36 )
(fig. S1).
Our fabricated TARC has high flexibility for
versatile surface adaption, as well as a micro-
scale structure consistent with the design (Fig.
2B). We examined the emittance switching
overtheentiresampleusingathermalinfra-
red(TIR)camera(Fig.2C).Weimagedthe
TARC surface together with two reference sam-
ples having similar thicknesses but constant
low thermal emittance (0.10, copper plate) or
constant high thermal emittance (0.95, black
tape), respectively. Although the thermal em-
ission of the reference samples appeared to
not be strongly temperature sensitive from 20
to 30°C, the TARC showed a marked change,RESEARCH | REPORTSSCIENCEscience.org 17 DECEMBER 2021•VOL 374 ISSUE 6574 1505
Fig. 1. TARC and its benefits for household thermal regulation.(A) Basic property of TARC in sky-window
(8 to 13mm) emittance modulation and schematics for temperature management when used as a household roof
coating. The data points are the measured sky-window emittances of a TARC. The two color bands represent
the temperature-independent thermal emittance of metals and radiative coolers. (B) TARC compared with other
thermal regulation systems, highlighting the unique benefit of TARC of being simultaneously energy-free and
temperature adaptive (details in table S1). (C) SCSESminof TARC compared with other existing roof-coating
materials for different cities representing the 15 climate zones in the United States. Red and blue circles indicate
positive and negative SCSESminvalues, respectively. The values are scaled to the area of the circles. Representation
of the triangle and circle icons is explained in the materials and methods (subsection,“Projection of energy savings”)( 36 ).